Gas discharge display panels, represented by a plasma display panel (herein after simply “PDP”), are display apparatuses that display images by light emission performed by exciting phosphors by means of ultraviolet light generated by gas discharge. According to the discharge forming method, PDPs are divided into two types of alternating current (AC) type and direct current (DC) type, where the AC type is most common because of superiority over the DC type in terms of brightness, light emission efficiency, and lifetime.
As is disclosed in Patent reference 1 for example, an AC-type PDP has the following structure. Two thin glass panels respectively provided with a plurality of electrodes (either display electrodes or address electrodes) and a dielectric layer are placed to oppose each other with a plurality of barrier ribs therebetween. A phosphor layer is provided so that phosphors are positioned between adjacent barrier ribs, thereby forming a plurality of discharge cells in matrix formation. The space between the two glass panels is filled with discharge gas. Furthermore, a protective layer (film) is provided on a surface of the dielectric layer covering the display electrodes.
While driving a PDP, power is supplied as necessary to the plurality of electrodes in a plurality of subfields that include an initialization period, an address period, a sustain period, and so on, according to a field time-sharing grayscale display method, thereby causing phosphor light emission by means of ultraviolet light generated by obtaining discharge in the discharge gas.
Here, a material for the protective layer provided for the front glass panel is required to generate discharge at a low discharge starting voltage while protecting the dielectric layer from ion bombardment incident to discharge at the same time. For this purpose, a material mainly made of magnesium oxide (MgO) is widely used for the protective layer of PDPs, as is disclosed in Patent reference 2, for MgO has an excellent sputtering resistant characteristic and a large secondary electron emission factor.
The conventional protective layer has the following problems.
The first problem is that conventional protective layers are susceptible to “discharge delay”. The discharge delay is a phenomenon caused in the address period, which specifically corresponds to a time lag from application of a pulse for address discharge to when actual discharge to take place. If the discharge delay is large, the possibility of preventing address discharge from occurring even at the end of the address pulse application becomes high, with which writing defect is likely caused. This phenomenon is more frequent in high-speed driving. The problem of discharge delay is a problem to be solved for improving image display performance of PDPs.
So as to counter this problem of discharge delay, a technology was already proposed to reduce the time lag by adding a predetermined amount of Si to MgO, as is disclosed in Patent references 3 and 7, for example. Furthermore, Patent reference 4 discloses a technology of attempting to reduce the time lag by adding a predetermined amount of H to the protective layer. Still further, Patent reference 5 discloses a technology of attempting to reduce the time lag by adding Ge.
The second problem is a characteristic change of the protective layer.
To be more specific, a surface of the protective layer is exposed in the discharge space. However the metal oxide film such as the MgO film has a characteristic that absorbs gas such as water (H2O) and carbon dioxide (CO2), which then would easily generate hydroxide compounds and carbonate compounds. In a process performed in the air from among the PDP manufacturing processes, a protective layer made of MgO tends to be contaminated by absorption of oil impurity, CO2, and H2O. When the absorption gas is absorbed by the surface of the MgO, the characteristic of the protective layer changes, thereby decreasing the secondary electron emission efficiency. As a result, the discharge starting voltage is raised, narrowing the driving margin of a PDP.
Furthermore, according to the level of absorption of gas for example by the protective layer, the discharge starting voltage is varied for each discharge cell. This would lead to a problem of display defect called “black noise” which specifically is a phenomenon in which accurate display of intended cells is impaired.
Therefore conventionally, the protective layer has a two-layer structure, as disclosed by Patent reference 6 for example, to improve quality and enhance stability. The disclosure specifically discloses a two-layer structure in which a second protection film is provided on a first protection film, where the first protection film has a comparatively excellent discharge characteristic and is (111) oriented, and the second protection film has such a film characteristic that hardly absorbs gas and has small moisture absorption, thereby attempting to prevent absorption of water molecules and impurity gas such as CO2.    Patent reference 1: Japanese Patent Publication No. H9-92133    Patent reference 2: Japanese Patent Publication No. H9-295894    Patent reference 3: Japanese Patent Publication No. H10-334809    Patent reference 4: Japanese Patent Publication No. 2002-33053    Patent reference 5: Japanese Patent Publication No. 2004-31264    Patent reference 6: Japanese Patent Publication No. 2003-22755    Patent reference 7: Japanese Patent Publication No. 2004-134407